For all of the Republican handwringing over President Obama's use—or overuse—of executive authority, there is one exclusive presidential power he has barely wielded at all: the veto.

Obama has rejected just two bills passed by Congress in his nearly six years in office. That's the fewest of any president since James Garfield, who didn't veto a single bill, but lasted only six months in the White House before his assassination in 1881. And when you look at presidents who have served as long as Obama has, you'd have to go back to James Monroe, the nation's fifth president, to find a chief executive who has formally clashed with Congress so rarely. Monroe vetoed just one bill in his two terms, according to records kept by the Senate.

That's about to change for Obama in January, when Republicans control both the House and the Senate. For the last four years, the president has issued dozens of veto threats, but Majority Leader Harry Reid has kept conservative House bills from reaching his desk.

The White House has not issued a formal veto threat on the bill, but spokesman Josh Earnest on Thursday noted the administration has taken "a dim view" on previous legislative proposals that have passed the GOP-led House but stalled in the Senate. At a press conference early Friday morning, Obama said both the legal challenges to the project in Nebraska and the ongoing State Department review of it should play out first, a position that Republicans have long equated to stalling.

"The president doesn’t have any more elections to win, and he has no other excuse for standing in the way," Speaker John Boehner said after the vote on Friday. "It’s time he start listening to the vast majority of Americans who support Keystone and help get more people back to work.”

The bill poses an interesting political question for Obama. He could choose to sign it as a way of helping Landrieu, but in a red state, even the passage of Keystone might not be enough to secure her victory over the favored Bill Cassidy, a Republican congressman who sponsored the House version of the bill. A presidential signature could also serve as an olive branch to Republicans and a modest gesture to voters who want the two parties to work together. But even if the president believes he'll eventually have to approve the project, a veto now would preserve the possibility of using Keystone to extract concessions from Republicans early next year. Signing the bill now, by contrast, would garner nothing in return, and it's unlikely Republicans would be able to piece together enough votes to override a veto even with their new members taking office in the new year. It would also anger environmentalists in the Democratic base who were cheering Obama's climate-change agreement with China, and who argue that the risks associated with Keystone's construction aren't worth the paltry number of permanent jobs that the State Department projects it would create.

So for the president, the political reasons for a veto probably outweigh the arguments for signing a Keystone bill this month. It would be only the third time Obama has used that power, but certainly not the last.

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Russell Berman is a staff writer at The Atlantic, where he covers politics.